These are Clipper Circuits.
1.(top left) these is negative clipping circuit with below reference clipping.
2.(bottom left) these is positive clipping circuit with above reference clipping.
3.(top right) these is negative clipping circuit with below reference clipping.
4.(bottom right) these is positive clipping with above reference clipping.
note: you can see the full wave form by turning off (graph of voltmeter) either both the left circuits or both the right circuits.
